The more expensive ones give you a better chance of winning, which is why they cost more (and they have larger purses). You can go to your state lottery website and they'll have all of the probabilities for each scratch off listed for you.
The way it's done here: you bring in a ticket, which the cashier doesn't even read, they scratch off a row of arbitrary numbers at the bottom, type it into their computer, it tells the winning amount and prints off a receipt for said amount. I've bought a ticket before with a $5 win. When I cashed it in, the cashier told me he looked it up when I bought it and saw that I was going to win. I don't know how he would have been able to do that without scratching anything, so he might've been feeding me some BS.